1. Field of the Invention
The present invention relates to a lighting device to be used suitably in, for example, a backlight of a liquid crystal display.
2. Description of Related Art
In recent years, with a remarkably enhanced efficiency and reduced cost of light emitting diodes (LEDs), LEDs have replaced conventional fluorescent tubes as dominant light sources for backlights of small and medium-sized liquid crystal displays, and further are gaining widespread use as light sources for large-screen liquid crystal displays or for general illumination.
In large-screen liquid crystal displays like liquid crystal TVs, direct type backlights have been used widely instead of edge light type backlights, which are commonly used in small and medium-sized liquid crystal displays. This is because such edge light type backlights have the following disadvantages.
An edge light type backlight is configured such that a light source is disposed at a lateral side of a light guide plate and optical films such as a prism sheet and a diffusing sheet further are disposed on the light emission side of the light guide plate. This configuration enables a reduction in the thickness of a display. On the other hand, in a large screen display, with an increase in the diagonal screen size, the required light quantity as well as the area of the display increase quadratically. However, the length of the lateral side, on which the light source can be disposed, increases linearly. As the screen size increases, the required light quantity density increases, which makes it more difficult to position the light source, and the heat generation density also increases, which makes it more difficult to dissipate the heat.
In that respect, since a direct type backlight is configured such that a large number of light sources are disposed in a plane beneath a liquid crystal panel and a diffusing plate and optical films such as a prism sheet and a diffusing film are disposed between the light sources and the liquid crystal panel, the required light quantity density does not change even when the screen size increases. Therefore, the heat generation density also remains unchanged, which is suitable for large screens.
In the case where an illumination of a flat surface with the above-mentioned direct type backlight or a desk lamp is performed by using approximate point light sources like LED light sources, when an angle from a perpendicular drawn from a light source to a flat surface to be illuminated is θ, and an intensity of light emitted in the θ direction is K(θ), the illuminance L(θ) at a point intersected by the illuminated surface in the θ direction is represented by the following equation:L(θ)=A×K(θ)×Cos3(θ)  (Equation 1)where A is a constant that depends on the distance from the light source to the illuminated surface.
Generally, a LED light source includes a LED element mounted on a substrate and a transparent resin that encapsulates the LED element. That is, the transparent resin forms a lens. When the encapsulating resin is formed in a hemispherical shape having its center at the LED element, the light emitted therefrom exhibits substantially Lambertian distribution. The Lambertian distribution is a light distribution characteristic such that a light ray emitted in the optical axis direction has the highest intensity K0 and a light ray emitted in a direction at an angle of θ with respect to the optical axis has a relative emission intensity K(θ)/K0 of Cos(θ).
When a flat surface is illuminated using such a Lambertian light source, the illuminance on the surface is represented by the following equation by substituting K(θ)=K0×Cos(θ) in Equation (1):L(θ)=A×K0×Cos4(θ)
Here, since the illuminance L0 in the optical axis direction is A×K0, the relative illuminance distribution L(θ)/L0 normalized with respect to the illuminance L0 in the optical axis direction is represented by the following equation:L(θ)/L0=Cos4(θ)FIG. 3 shows the relative illuminance distribution.
As shown in FIG. 3, the illuminance on the illuminated surface decreases sharply with increasing angle. Here, when the distance from the light source to the illuminated surface is D, the angle θ of light emitted to a position on the surface with a distance x from the optical axis is represented by the following equation:Tan(θ)=x/D 
Accordingly, if the horizontal axis of the graph of FIG. 3 is redefined as the distance x=D×Tan(θ), the resulting graph indicates the spatial illuminance distribution, in which the position on the optical axis is brightest and becomes darker suddenly with increasing distance from the optical axis.
Since the LED light sources used for a direct type backlight are required to have a characteristic of illuminating a largest possible area uniformly with a smallest possible number of LED light sources, the characteristic as mentioned above is not desirable for such a backlight. The characteristic of illuminating a specific area uniformly may be effective for use in, for example, a desk lamp, etc., in addition to a backlight.
The condition for illuminating a flat surface uniformly using a single point light source is that the right-hand side of Equation 1 has a constant value L0 irrespective of the angle θ, that is, the following equation is satisfied.A×K(θ)×Cos3(θ)=L0By transforming the above equation, the following equation is obtained:K(θ)=L0/A×Cos−3(θ)
In the above equation, L0/A is the light intensity at the angle θ of 0 degree, that is, the axial light intensity. The light distribution characteristic for achieving a uniform illuminance distribution is obtained when the relative light intensity distribution K(θ)/K0 normalized with respect to the axial light intensity K0 satisfies following equation:K(θ)/K0=Cos−3(θ)  Equation (2)FIG. 4 shows the emitted light intensity distribution.
As is clear from FIG. 4, the light intensity required for the uniform illumination increases sharply with increasing angle. Therefore, it is impossible to satisfy the above Equation 2 in the entire range of angles from −90 degrees to +90 degrees, and thus the goal is to obtain the characteristic similar to the characteristic represented by Equation 2 in as wide a range as possible.
For this goal, various shapes of encapsulating transparent resins have been proposed to improve the light distribution characteristics by utilizing the refraction and reflection at the surfaces of the encapsulating transparent resins. For example, JP 2006-092983 A discloses a shape of an encapsulating resin having a concave around the optical axis and a convex extending outwardly from the concave.
It is, however, not easy to obtain the characteristic of increasing the emitted light intensity sharply with increasing angle as shown in FIG. 4. In particular, the difficulty of obtaining the above-mentioned characteristic increases further if a LED of limited size and an encapsulating resin of limited size are used.